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Offputting smell Diesel exhaust fumes alter the flowery smells that guide bees when they forage, potentially sending them off course and putting the food-growing industry at risk, a study has found.

Honeybees rely heavily on their sense of smell to locate flowers from which they harvest life-giving nectar -- transferring pollen grains from one bloom to another in the process.

The new research, published in Nature journal Scientific Reports shows that diesel exhaust fumes from cars, tractors or power generators can chemically alter the smell of flowers and render them undetectable to bees.

This, in turn, threatens the insects' crucial role as key pollinators of human food crops.

"Somewhere in the region of 70 per cent of world crops require pollination services, and... about 35 per cent of our current food production is reliant on pollination," says study co-author Tracey Newman of the University of Southampton.

For the study, Newman and a team created a synthetic odour blend mimicking the complex chemical mix that makes up the smell of oilseed rape flowers.

The synthetic blend of eight chemicals was released into a sealed glass vessel with clean air, and another that contained diesel exhaust at levels similar to rush-hour, roadside fumes.

The fumes contained high concentrations of NOx gases: nitric oxide and nitrogen dioxide, as well as carbon monoxide.

Within one minute, the chemicals alpha-farnesene and alpha-terpinene, which comprised 72.5 per cent and 0.8 per cent of the blend respectively, were "rendered undetectable" in the diesel-polluted air.

The other chemicals were also considerably reduced in volume, while there was no change for the blend in the clean-air vial.

Altered senses

Next, the team tested whether bees would notice the difference.

They trained the insects by exposing them to the eight-chemical synthetic odour blend and feeding them a sugar solution at the same time to build an association of reward -- as the smell of flowers hold the promise of nectar.

Over time, the trained bees would start sticking out their tongue-like proboscis in anticipation whenever they recognised the odour.

The scientists then removed chemical elements from the synthetic odour to create a depleted mix like the one left over after diesel exposure.

When they removed alpha-terpinene, the insects' ability to recognise the odour dropped to less than 30 per cent, said Newman -- demonstrated by the bees no longer extending their proboscis.

When alpha-farnesene was taken out too, the ability dropped even further.

"This isn't just about a bee getting confused because there is a new smell around. This is actually that the chemistry of the odour itself is being chemically altered," she explains.

If the foraging bees are unable to find nectar, the entire hive will suffer for a lack of food -- as will the plants that depend on pollination to reproduce.

"And without efficient, effective pollination, there are going to be consequences for human health," saysNewman.

Bees account for some 80 per cent of pollination by insects, but their numbers have slumped in Europe and the United States in the past 15-odd years due to a worrying phenomenon dubbed colony collapse disorder (CCD).

The mysterious plague, often characterised by a rapid loss of adult worker bees, has been blamed on everything from agricultural pesticide use, a loss of wild bee habitat, a virus or fungus, mites -- or a combination that may now also include diesel fumes.

The disorder has killed off about 30 per cent of bees annually since 2007.